In eukaryotic cells DNA is packaged with histones to form chromatin. Approximately 150 base pairs of DNA are wrapped twice around an octamer of histones (two each of histones 2A, 2B, 3 and 4) to form a nucleosome, the basic unit of chromatin. Changes in the ordered structure of chromatin can lead to alterations in transcription of associated genes. This process is highly controlled because changes in gene expression patterns can profoundly affect fundamental cellular processes, such as differentiation, proliferation and apoptosis. Control of changes in chromatin structure (and hence of transcription) is mediated by covalent modifications to histones, most notably of their N-terminal tails. These modifications are often referred to as epigenetic because they can lead to heritable changes in gene expression, but they do not affect the sequence of the DNA itself. Covalent modifications (for example, methylation, acetylation, phosphorylation and ubiquitination) of the side chains of amino acids are enzymatically mediated.
The selective addition of methyl groups to specific amino acid sites on histones is controlled by the action of a unique family of enzymes known as histone methyltransferases (HMTs). The level of expression of a particular gene is influenced by the presence or absence of one or more methyl groups at a relevant histone site. The specific effect of a methyl group at a particular histone site persists until the methyl group is removed by a histone demethylase, or until the modified histone is replaced through nucleosome turnover. In a like manner, other enzyme classes can decorate DNA and histones with other chemical species, and still other enzymes can remove these species to provide control of gene expression.
The orchestrated collection of biochemical systems behind transcriptional regulation must be tightly controlled in order for cell growth and differentiation to proceed optimally. Disease states result when these controls are disrupted by aberrant expression and/or activity of the enzymes responsible for DNA and histone modification. In human cancers, for example, there is a growing body of evidence to suggest that dysregulated epigenetic enzyme activity contributes to the uncontrolled cell proliferation associated with cancer as well as other cancer-relevant phenotypes such as enhanced cell migration and invasion. Beyond cancer, there is growing evidence for a role of epigenetic enzymes in a number of other human diseases, including metabolic diseases (such as diabetes), inflammatory diseases (such as Crohn's disease), neurodegenerative diseases (such as Alzheimer's disease), and cardiovascular diseases. Therefore, selectively modulating the aberrant action of epigenetic enzymes holds great promise for the treatment of a range of diseases.
Histone Methyltransferase EZH2
Polycomb group (PcG) and trithorax group (trxG) proteins are known to be part of the cellular memory system. Francis et al. (2001) Nat Rev Mol Cell Biol 2:409-21; Simon et al. (2002) Curr Opin Genet Dev 12:210-8. Both groups of proteins are involved in maintaining the spatial patterns of homeotic box (Hox) gene expression, which are established early in embryonic development by transiently expressed segmentation genes. In general, PcG proteins are transcriptional repressors that maintain the “off state,” and trxG proteins are transcriptional activators that maintain the “on state.” Because members of PcG and trxG proteins contain intrinsic histone methyltransferase (HMTase) activity, PcG and trxG proteins may participate in cellular memory through methylation of core histones. Beisel et al. (2002) Nature 419:857-62; Cao et al. (2002) Science 298:1039-43; Czermin et al. (2002) Cell 111:185-96; Kuzmichev et al. (2002) Genes Dev 16:2893-905; Milne et al. (2002) Mol Cell 10:1107-17; Muller et al. (2002) Cell 111:197-208; Nakamura et al. (2002) Mol Cell 10:1119-28.
Biochemical and genetic studies have provided evidence that Drosophila PcG proteins function in at least two distinct protein complexes, the Polycomb repressive complex 1 (PRC1) and the ESC-E(Z) complex (also known as Polycomb repressive complex 2 (PRC2)), although the compositions of the complexes may be dynamic. Otte et al. (2003) Curr Opin Genet Dev 13:448-54. Studies in Drosophila (Czermin et al. (supra); Muller et al. (supra)) and mammalian cells (Cao et al. (supra); Kuzmichev et al. (supra)) have demonstrated that the ESC-E(Z)/EED-EZH2 (i.e., PRC2) complexes have intrinsic histone methyltransferase activity. Although the compositions of the complexes isolated by different groups are slightly different, they generally contain EED, EZH2, SUZ12, and RbAp48 or Drosophila homologs thereof. However, a reconstituted complex comprising only EED, EZH2, and SUZ12 retains histone methyltransferase activity for lysine 27 of histone H3. U.S. Pat. No. 7,563,589 (incorporated by reference).
Of the various proteins making up PRC2 complexes, EZH2 (Enhancer of Zeste Homolog 2) is the catalytic subunit. The catalytic site of EZH2 in turn is present within a SET domain, a highly conserved sequence motif (named after Su(var)3-9, Enhancer of Zeste, Trithorax) that is found in several chromatin-associated proteins, including members of both the Trithorax group and Polycomb group. SET domain is characteristic of all known histone lysine methyltransferases except the H3-K79 methyltransferase DOT1.
In addition to Hox gene silencing, PRC2-mediated histone H3-K27 methylation has been shown to participate in X-inactivation. Plath et al. (2003) Science 300:131-5; Silva et al. (2003) Dev Cell 4:481-95. Recruitment of the PRC2 complex to Xi and subsequent trimethylation on histone H3-K27 occurs during the initiation stage of X-inactivation and is dependent on Xist RNA. Furthermore, EZH2 and its associated histone H3-K27 methyltransferase activity was found to mark differentially the pluripotent epiblast cells and the differentiated trophectoderm. Erhardt et al. (2003) Development 130:4235-48).
Consistent with a role of EZH2 in maintaining the epigenetic modification patterns of pluripotent epiblast cells, Cre-mediated deletion of EZH2 results in loss of histone H3-K27 methylation in the cells. Erhardt et al. (supra). Further, studies in prostate and breast cancer cell lines and tissues have revealed a strong correlation between the levels of EZH2 and SUZ12 and the invasiveness of these cancers (Bracken et al. (2003) EMBO J 22:5323-35; Kirmizis et al. (2003) Mol Cancer Ther 2:113-21; Kleer et al. (2003) Proc Natl Acad Sci USA 100:11606-11; Varambally et al. (2002) Nature 419:624-9), indicating that dysfunction of the PRC2 complex may contribute to cancer.
Recently, somatic mutations of tyrosine 641 (Y641F, Y641N, Y641S and Y641H) of EZH2 were reported to be associated with follicular lymphoma (FL) and the germinal center B cell-like (GCB) subtype of diffuse large B-cell lymphoma (DLBCL). Morin et al. (2010) Nat Genet 42:181-5. In all cases, occurrence of the mutant EZH2 gene was found to be heterozygous, and expression of both wild-type and mutant alleles was detected in the mutant samples profiled by transcriptome sequencing. It was also demonstrated that all of the mutant forms of EZH2 could be incorporated into the multi-protein PRC2 complex, but that the resulting complexes lacked the ability to catalyze methylation of the H3-K27 equivalent residue of a peptidic substrate. Hence, it was concluded that the disease-associated changes at Tyr641 of EZH2 resulted in loss of function with respect to EZH2-catalyzed H3-K27 methylation.